1. Field of the Invention
This invention relates to superluminescent diodes. In particular, it relates to angled-stripe devices and methods of stabilizing the wavelengths of these broad-bandwidth optical sources.
2. Description of the Related Art
The design of an interferometric fiber-optic sensing system depends on the stability of the wavelength of the optical source. In particular, for the fiber-optic rotation sensors to be useful in navigation, the emission wavelength of the optical source must be controlled to one part in a million or better.
Wavelength stabilization of the optical source has been accomplished by using a thermally-compensated optical interferometer to detect changes in the emission wavelength of the source. In a thermally-compensated interferometer, the interferometer is designed so that phase shifts occur in the interferometer only in response to shifts in the wavelength of the incident light and not in response to changes in ambient temperature. Shifts in the source wavelength generate an error signal in the interferometer. This signal is used to correct the emission wavelength of the source.
The light source most suitable for the fiber-optic rotation sensor is the superluminescent laser diode or SLD. The SLD emits in a broad spectral band which reduces phase noise in the sensor caused by the Kerr effect and by coherent backscatter in the fiber. The SLD most often used for rotation sensors is a device which is a modified laser diode. In the SLD, the optical feedback present in laser diodes is suppressed by either anti-reflection coating of the laser diode facets, or by truncating the current stripe on the diode so that there is a light-absorbing region between the end of the truncated stripe and the rear facet of the diode. Because of the absence of feedback, these devices emit only incoherent light, the type of light that is required by the fiber-optic rotation sensor.
Although the SLD's fabricated by either the truncated current-stripe method or with anti-reflection coatings are appropriate as sources for fiber-optic rotation sensors, high accuracy control of the emission wavelength of these devices is made difficult by the short coherence length of the source. For these devices, this length is typically on the order of 20 um to 60 um. To generate an error signal in response to changes in the emission wavelength of the source, interferometers must therefore be designed and fabricated with optical pathlength differences (opds) on the order of the coherence length of the SLD.
The design and fabrication of short opd interferometers is challenging and expensive. In addition, because the sensitivity of an interferometer to wavelength changes is proportional to the opd between the interfering optical waves, it is most desirable to use thermally-compensated interferometers with large opds that are inexpensive and easy to fabricate.
Because of its structure, the angled-stripe SLD permits feedback in a restricted section of its waveguide channel as well as incoherent emission in the direction parallel to the stripe. These two light beams are spatially separated, with the coherent output normal to the air/GaAs (gallium arsenide) chip interface and the incoherent refracting into the air at an angle determined by the angle between the stripe and the normal to the chip.
The emitted coherent light has a narrow bandwidth, and hence, a large coherence length which is necessary for high accuracy sensing of wavelength shifts in the source emission. The incoherent output is broadband and appropriate for use in the fiber-optic rotation sensor.
The shifts in the emission wavelength of the source are caused by changes in either the injection current to the device or in the device temperature. Such changes will result in wavelength shifts in both the coherent and the incoherent light beams.
In the angled-stripe configuration, because the coherent and incoherent emissions share the same active region, the temperature in the coherent light region is the same as that in the incoherent light region. Therefore, temperature changes in the active region will result in the same wavelength shifts in both the coherent and incoherent light emissions. These shifts are detected by a highly sensitive, large-opd-interferometer, and are used to generate an error signal which is used to control the temperature and/or the injection current in the active region of the SLD.